Various teachings about active retrodirective systems have been described in prior publications. The teachings of the present application can be better understood with reference to these prior publications:    (1) L. C. Van Atta, U.S. Pat. No. 2,908,002, 1959    (2) S. N. Andre and D. J. Leonard, IEEE Trans. Ant. and Prop., March 1964, pp. 181-186    (3) C. Y. Pon, IEEE Trans. Ant. and Prop., March 1964, pp. 176-180.    (4) P. Horowitz, and W. Hill, “The Art of Electronics”, Cambridge University Press, 1980.    (5) S. Haykin, “Communication Systems” 3rd Ed. (John Wiley, New York, 1994), Sec. 2.8.    (6) J. W. Goodman, “Introduction to Fourier Optics, 2nd Ed (McGraw Hill, New York, 1996).    (7) R. Y. Miyamoto, Y. Qian, and T. Itoh, IEEE 1999 MTT-S Digest, pp. 655-658.    (8) L. D. DiDomenico, and G. M. Rebeiz, IEEE Trans on Microwave Theory & Tech, vol. 49, no. 4, pp. 677-84 (2001)    (9) M. Dawood, and R. M. Narayanan, IEEE Trans on Aero & Electronic Systems, vol. 37, no. 2, April 2001, pp. 586-94    (10) B. Y. Toh, V. F. Fusco, and N. B. Buchanan, IEEE Trans Ant. and Prop, vol. 50, no. 10, pp. 1425-1432, October 2002    (11) S. Gupta and E. R. Brown, 2003 IEEE MTT-S Digest (IEEE, New York, 2003), pp. 599-603 (2003).    (12) E. R. Brown, A. G. Cotler, A. Umali, and S. Gupta, 2004 IEEE MTT-S Digest, pp. 751-754. [2004].    (13) E. B. Brown and E. R. Brown, IEEE 2005 IMS Digest, paper TH3B-3.    (14) E. R. Brown, “Retrodirective Noise Correlating Radar: Methods and Apparatus,” U.S. patent application Ser. No. 11/043,745, 2005.Each of these publications is incorporated herein by reference as if set forth in full herein.
Various retrodirective system methods and apparatus have been used or proposed in the past. A retrodirective antenna array for use as an electromagnetic reflector was described by Van Atta in 1959, in U.S. Pat. No. 2,908,002, using feedhorn-type antennas. Van Atta showed how the arrangement of transmit and receive antenna arrays should occur symmetrically about a geometric center point, and how the retrodirective re-transmission of received radiation would occur automatically if the time delay between the symmetric pairs was equal. However, the teachings of Van Atta were strictly directed to a passive reflector component for use in radar or communications. Van Atta did not address the integration of the retrodirective array to form a radar or communications system by the addition of active (gain) electronics between each receive antenna element and the symmetric transmit element.
Electronic gain and other components were first added to each channel of Van-Atta retrodirective antenna arrays in the early 1960s, and applied to various communications systems, particularly for satellites [S. N. Andre and D. J. Leonard, IEEE Trans. Antennas and Propagation, March 1964, pp. 181-186]. The primary application was in transponders whereby the satellite system retransmits the incident signal in the same direction from where it originated but with amplified power and, perhaps, an offset carrier frequency.
It was recognized early on that the Van Atta array was somewhat impractical for communications because it requires separate receive and transmit antennas. So an alternative type of retrodirective system was proposed and demonstrated by C. Y. Pon [IEEE Trans. Antennas and Propagation, March 1964, pp. 176-180] that required only one antenna for receive and transmit. It utilized a heterodyne electronic channel connecting the common transmit/receive antenna. Retrodirectivity is achieved by multiplying the incoming signal against a local oscillator at twice the frequency of the signal.
The 1970s and 80s saw very little advancement in the technology or application of retrodirective antennas in RF systems. In the 1990s, interest was rejuvenated with advancements in microwave integrated circuits and semiconductor devices that allowed planar antennas (e.g., patches) to be combined with mixers and local oscillators in very efficient and compact circuits and systems. This work was aimed at communications applications, and implemented primarily the Pon retrodirective technique summarized above.
In 2002 research began at UCLA in the application of retrodirective antennas toward a radar system. The initial idea was to use the Van Atta architecture since it provides much greater isolation between transmit and receive than the Pon architecture, and radar generally requires much more isolation than a communications system. One array was designed to transmit and the other array to receive. Additive white Gaussian noise (AWGN) was investigated first because of its prevalence in all electronics.
The retrodirective noise correlating (RNC) radar was first analyzed in 2003 [Gupta and Brown, 2003] and then demonstrated in 2004 [Brown, Cotler and Gupta] in the S band. Its key feature was direct RF feedback between the receive and transmit arrays of a Van-Atta antenna configuration. This allowed for ultrafast detection of targets on a time scale corresponding to a few round trips through free space. Target angle was determined by cross-correlation between neighboring antennas, just as in radio astronomy receivers. Some features of this initial attempt at a retrodirective noise correlating (RNC) system were set forth in U.S. patent application Ser. No. 11/043,745, now abandoned.
While representing the first known application of a retrodirective active antenna to target sensing, the RNC system was fraught with problems that precluded its application in practical systems. First, it was observed early on that the RNC system could not distinguish real targets from stationary clutter. This is because of its inherent incoherence and, therefore, its inability to distinguish targets based on motion and the associated Doppler shift in the frequency domain.
A second problem with the RNC system was its inability to unambiguously determine target range.
A third problem with the RNC system was its limited spatial resolution, determined by the element-to-element spacing rather than the entire antenna aperture. This was a result of the incoherence of the detection process by which the absolute phase of the incoming waveform was not determined.
Because of these shortcomings, a need still exists to create an active retrodirective system that can provide range and bearing estimation, and also rejection of stationary clutter, and thus allowing such systems to function as effective radar. These needs are especially important for short range applications that can automatically detect, track, and acquire a target without the need for a separate sensor to provide cueing.
A need also exists in the field for sensors that can detect very small targets, such as ballistic projectiles, moving very fast and at close range. In such systems, the detection and acquisition times of the radar must be short compared to the time-of-flight of such projectiles.